The present invention relates to signal generators, and more particularly to a television signal generator which is fully digital, uses a single system clock and a single digital to analog converter.
Prior television test signal generators, such as the 1410R Series sync and test signal generators manufactured by Tektronix, Inc. of Beaverton, Oregon, were analog. All of the signal components are generated with analog techniques. Digital circuitry is used only to time and switch the analog components. Some of the problems with this architecture include imprecise subcarrier to horizontal (SCH) phase and residual subcarrier in the video output. However such analog techniques did allow for slewing of phase so that ringing of multiburst signals, caused typically by filters in a television system being tested, could be readily detected.
Another approach, as exemplified by the PM5630 test signal generator manufactured by Philips Denmark and the TSG300 component television generator manufactured by Tektronix, Inc., uses a digital component approach The analog component generators are replaced with programmable read only memories (PROMs) followed by digital to analog converters (DACs) and low pass filters. The chrominance subcarrier is generated by modulating an oscillator running at the subcarrier frequency with the outputs of the chrominance component envelop DACs. The output of the luminance channel is summed with the modulated subcarrier to produce the test signal output. This architecture provides improved test signal flexibility, but the analog modulation scheme still leaves residual subcarrier and SCH error in the video output. Also due to the memory required to store separate patterns for each horizontal line to simulate phase slewing, such phase slewing capability is lost.
What is desired is a television signal generator which eliminates residual subcarrier and SCH error in the video output while maintaining a phase slewing capability without large memory expenditure.